XRB is the formal ticker and designation under the X-type supernational currencies.
"M" prefix is a SI prefix, another way to write it would be XRB (millions). The reason I chose these SI dividers is I think currency amounts without decimals are easier to use and write. Anticipating a wider user-base and depreciation over time as keys are lost it seemed most likely people would eventually be using kxrb or plain xrb when it comes in to wide use.
The wallet defaults to Mxrb and all exchanges are listing it in millions right now.
All the dividers are listed in code and on the GitHub wiki:
https://github.com/clemahieu/raiblocks/blob/master/rai/utility.hpp
https://github.com/clemahieu/raiblocks/wiki/Distribution-and-Mining
Thanks for the explanation, if not wanting decimals was part of your design then it makes sense to have 1xrb = 1rai.
It's just a matter of preference I guess (I prefer 1xrb = 1raiblock = 1,000,000rai ), there are currencies out there like japanese yen which have no decimals.